Many ISP's block SMTP port 25 to try to control SPAM traffic.
Try to use port 465 or 587 (SSL/TLS accept all certificates).
Also remember to use the same credentials for SMTP authentication that you used with
IMAP account. If you use your ISP's SMTP server you won't see "Sent" emails on Webmail.

Unless you have bought a certificate for your email server address,
You should configure your App to use above mentioned "real" addresses
rather than alias addresses (like mail.yourdomain.com.) (See picture.)

5. What if I my email stops working?

You can always remove your account from your App and reconfigure it from scratch.
If you use IMAP account the emails will stay safe on our servers even if your device
changes.